The present invention pertains to an indicator system which visually displays the time-temperature integral to which a product has been exposed.
The desirability of detecting whether or not a frozen product has been allowed to thaw has long been recognized and numerous tell-tale devices are described in the literature. One class of these relies upon material which is frozen but which melts at some preselected temperature so as to irreversibly activate an indicator, either chemically or physically. Typically of these devices are those described in the following U.S. patents:
______________________________________ Nos. 1,917,048 Nos. 2,753,270 Nos. 2,955,942 2,216,127 2,762,711 3,047,405 2,277,278 2,788,282 3,055,759 2,340,337 2,823,131 3,065,083 2,553,369 2,850,393 3,194,669 2,617,734 2,852,394 3,362,834 2,662,018 2,951,405 3,437,010 ______________________________________
All of the above devices merely signal "thaw" with no attempt to measure the period during which the product is thawed or the temperature which the product attains while thawed.
A second class of known indicators utilizes diffusion or capillary action of a liquid on a wick or similar permeable member. These devices while often cumbersome, provide some degree of gradation and are typified by the devices of the following U.S. Patents:
______________________________________ Nos. 2,560,537 Nos. 3,243,303 2,716,065 3,414,415 2,951,764 3,479,877 3,118,774 ______________________________________
The majority of the prior art devices however are directed primarily at the phenomenon of thawing and the attendant damage which occurs. It is now recognized that various natural and synthetic materials deteriorate with the passage of time even when taking the precaution of storing under adequate refrigeration. This is true even with such additional or alternative precautions as packaging in an inert atmosphere, sterilization or adding spoilage retardants. Thus, for example, foods, films, pharmaceuticals, biological preparations and the like, can demonstrate decomposition with the passage of time, even when sterilized or maintained at sufficiently low temperatures to preclude microbiological degradation. Such decomposition occurs for various reasons, including strictly chemical reactions, such as oxidation, and enzymatic processes. Frozen foods and ice cream show deterioration even when held in a frozen state. A system which would monitor such decomposition or deterioration would be extremely valuable. The deterioration kinetics involved in such processes however, are exceedingly complex. For example, while it is clear that deterioration is a function of temperature, the rate of this deterioration of such products can also vary with temperature. One rate of deterioration will exist at a first temperature while a different rate obtains at a second temperature. The total amount of deterioration will depend upon the time at which the product is held at each temperature; i.e., the integral of time and temperature.
The quotient of (a) the rate of change at one temperature of an article's property whose deterioration is being monitored to (b) the rate of change at a lower temperature is often expressed for ten degree increments and represented by the symbol "Q.sub.10 " for the Celsius scale and "q.sub.10 " for the Fahrenheit scale. This quotient is substantially constant within limited temperature ranges.
The practical effect of the foregoing can be seen for example from two comparable samples of frozen food which are processed and packaged at the same time. If in the course of distribution or storage one package is allowed to rise in temperature by 10.degree. or 20.degree. C., even without thawing, its life will be reduced as compared with the other package which was maintained at a lower temperature for its entire storage life since the rate of decomposition of the contents of the first package is accelerated during the storage at the higher temperature. A consumer about to purchase these packages, both of which are now stored at normal freezer temperature, has no way of ascertaining this difference in temperature histories.
Systems have been suggested for monitoring the temperature history of a product. Thus U.S. Pat. No. 2,671,028 utilizes an enzyme such as pepsin in indicator systems while U.S. Pat. No. 3,751,382 discloses an enzymatic indicator in which urease decomposes urea with the reaction products causing a change in the pH of the system. The activity of the enzyme, and thus rate of decomposition, is dependent on temperature so that the change in pH resulting from this decomposition can be monitored by conventional acid-base indicators. This type of system, which appears to be directed at the specific problem of microbiological putrefaction rather than the broader problem of monitoring temperature histories, suffers from the inherent limitation of any enzymatic reaction. Thus while enzyme activity is a function of temperature, it is also sensitive to the very passage of time being measured, enzymatic activity generally decreasing with time. Enzyme activity to pH change and such change is the operative factor in, for example, the system of U.S. Pat. No. 3,751,382. A more sophisticated system is described in U.S. Pat. No. 3,768,976 in which time temperature integration is achieved by monitoring permeation of oxygen from the atmosphere through a film, utilizing a redox dye to provide a visual read out. This device is however dependent upon the presence of atmospheric oxygen and somewhat cumbersome in configuration and dimensions.
A further problem is that the change in rate of quality loss per unit of temperature change differs for different products. Thus the change in the rate of deterioration per unit of temperature change for certain fruits and berries is vastly different from the change in rate for lean meats. The values for dairy products are different from both. For example, within the range of 0.degree. to -20.degree. C., raw fatty meat and pre-cooked fatty meat have Q.sub.10 's of about 3, whereas raw lean meat and pre-cooked lean meat have Q.sub.10 's between 5 and 6. Vegetables generally have a Q.sub.10 of between 7 and 8, whereas fruits and berries have a Q.sub.10 of approximately 13. Consequently, a system which is dependent on a single enzymatic reaction or the permeability of a given film will be suitable as an indicator only for those materials having a similar slope for their relationship of change of rate of decomposition to temperature. Although U.S. Pat. No. 3,751,382 describes a method for modifying the time at which the indicator's color change occurs, the activation energy of the enzyme system is modified only slightly and the ratio of change in reaction rate per temperature unit remains substantially the same. The same is true of the device described in U.S. Pat. No. 3,768,976 which is dependent solely on gas permeability.
U.S. Pat. application Ser. No. 469,851 filed on May 14, 1974, now U.S. Pat. No. 3,946,611, which application is incorporated herein by reference, teaches a novel indicator system which overcomes the aforementioned prior art problems. The device comprises a gas impermeable envelope in which is enclosed a gas generating means, an indicating means comprising a wick impregnated with a pH sensitive dye and a barrier comprising a rate controlling film to control the rate at which gas leaving the chamber in which it is enclosed passes into the compartment which contains the wicking means. The envelope of the device is separated into two compartments, a first gas generating compartment and a second wicking means compartment by a cross seal which although it isolates one chamber from the other is only a partial seal in that one end of the wicking means protrudes into the gas generating compartment. The function of the cross seal is to force the gas generated to move up the wicking means by wicking up the active length and to prevent gas from migrating to the far end of the wick by channeling along the wick.
Another suitable configuration for this device is taught in U.S. Pat. application Ser. No. 515,165 filed on Oct. 16, 1974, now U.S. Pat. No. 3,932,134 incorporated herein by reference. A first compartment for the wick by sealing device utilizes the same components as the earlier described device of U.S. patent application Ser. No. 469,851 now U.S. Pat. No. 3,946,611. However, the gas impermeable envelope is divided into the gas generating chamber and wicking chamber by utilizing a peripheral seal which seals the envelope along the two long sides of the wick and one end. The seal then forms a compartment of larger dimension in which the gas generating means is contained and into which one of the wick protrudes. The structure of these devices are described more fully below.